1. Field of The Invention
The present invention relates to the control of fuel injection in internal combustion engines, and more particularly to a method of learning control for air/fuel ratio of an internal combustion engine operated with fuel injection to maintain the air/fuel ratio at the stoichiometric value.
2. Description of the Prior Art
In an internal combustion engine equipped with a three way catalytic converter it is very important that the air/fuel ratio in the operation of the engine should be maintained exactly at the stoichiometric value. To meet with such a requirement, particularly in an internal combustion engine operated with fuel injection, it is known to control the fuel injection by the so-called learning control method. According to a known method of learning control for air/fuel ratio of an internal combustion engine operated with fuel injection the fuel injection is controlled to maintain the air/fuel ratio at the stoichiometric value according to a basic fuel injection time duration based upon engine load and engine revolution speed, an air/fuel ratio feedback complementing factor which is changed over between a constantly increasing mode which continues as long as the air/fuel ratio is on the lean side of the stoichiometric value and a constantly decreasing mode which continues as long as the air/fuel ratio is on the rich side of the stoichiometric value, and a learning control factor. The actual fuel injection time duration is obtained by multiplying said basic fuel injection time duration by said air/fuel ratio feedback complementing factor and by further modifying the product by said learning control factor, so as to increase the actual fuel injection time duration when said learning control factor increases or to decrease the actual fuel injection time duration when said learning control factor decreases.
The learning control factor is obtained by initially setting its value at a predetermined initial value, and then calculating a mean value of each two successive end values of said air/fuel ratio complementing factor in said constantly increasing and constantly decreasing modes, thereby increasing the current value of the learning control factor by a predetermined increment each time when said mean value is calculated to be larger than a predetermined first limit value, decreasing the current value of the learning control factor by a predetermined decrement each time when said mean value is calculated to be smaller than a predetermined second limit value which is smaller than said first limit value, and maintaining the current value of the learning control factor each time when said mean value is calculated to be between said first and second limit values. Such a progress of the air/fuel ratio feedback complementing factor is shown in FIG. 1 of the accompanying drawings in relation to the fluctuation of the air/fuel ratio to both sides of the stoichiometric value.
However, when for some reasons such as a malfunction of a part of the fuel injection control device the learning control factor has been reduced for a substantial amount when the engine is operating on the lean side of the stoichiometric condition, due to a substantial decrease of the amount of fuel injected into the engine, the engine continues to operate on the lean side of the stoichiometric value, and therefore the air/fuel ratio feedback complementing factor continues to increase, finally reaching its upper limit value and saturating as a constant value, so that the next expected end value of the air/fuel ratio feedback complementing factor in the constantly increasing mode is not obtained, whereby the operation of the engine is stuck everlastingly to the lease side of the stoichiometric condition. These performances of the air/fuel ratio, the air/fuel ratio feedback complementing factor, and the learning control factor are shown in FIG. 2 of the accompanying drawings.
The similar sticking of the engine operation to the rich side of the stoichiometric condition may also occur if the learning control factor increases so much due to a malfunction of the fuel injection control device when the engine is operating on the rich side of the stoichiometric condition that the air/fuel ratio feedback complementing factor decreases down to its lower limit before the actual fuel injection time duration is increased enough to change over the operation of the engine from the rich side to the lean side of the stoichiometric condition.